RECORD: Darwin, C. R. 1845. Extracts from letters to the General Secretary, on the analogy of the structure of some volcanic rocks with that of glaciers. By C. Darwin, Esq., F.R.S. Specimens were exhibited. With observations on the same subject by Prof. Forbes. [Communicated by J. D. Forbes. 3 February] Proceedings of the Royal Society of Edinburgh 2: 17-18.

2. Extracts from Letters to the General Secretary, on the Analogy
of the Structure of Some Volcanic Rocks with that of Glaciers. By C.
Darwin, Esq., F.R.S. Specimens were exhibited. With Observations on the
same subject, made by Professor Forbes.1

"I take the liberty of addressing you, knowing how much you are
interested on the subject of your discovery of the veined structure of
glacier ice. I have a specimen (from Mr. Stokes's2 collection) of
Mexican obsidian, which, judging from your description, must resemble,
to a considerable degree, the zoned ice. It is zoned with quite
straight parallel lines, like an agate; and these zones, as far as I
can see under the microscope, appear entirely due to the greater or
lesser number of excessively minute, flattened air cavities. I cannot
avoid suspecting that in this case, and in many others, in which lava
of the trachytic series (generally of very imperfect fluidity) are
laminated, that the structure is due to the stretching of the mass or
stream during its movement, as in the ice-streams of glaciers. * * *

"If the subject of the lamination of volcanic rocks
should interest you, I would venture to ask you to refer to p. 65-72 of
my small volume of 'Geological Observations on Volcanic Islands.'* I
there

* The laminated, volcanic rocks of Ascension,
consist, as described by Mr. Darwin, of excessively thin, quite
parallel layers of minute crystals of quartz (determined by Professor
Miller3) and diopside; of atoms of an oxide of iron, and of an
amorphous, black augitic mineral; and, lastly, of a more or less pure
feldspathic stone, with perfect crystals of feldspar placed lengthways.
The following is a portion of the passage referred to:—"Several causes
appear capable of producing zones of different tension in masses
semiliquified by heat. In a fragment of devitrified glass I have
observed layers of sphærulites, which appeared, from the manner in
which they were abruptly bent, to have been produced by the simple
contraction of the mass in the vessel, in which it cooled. In certain
dykes on Mount Ætna, described by M. Elie de Beaumont,4 as bordered by
alternating bands of scoriaceous and compact rock, one is led to
suppose that the stretching movement of the surrounding strata, which
originally produced the fissures, continued, whilst the injected rock
remained fluid. Guided, however, by Professor Forbes's clear
description of the zoned structure of glacier ice, far the most
probable explanation of the laminated structure of these feldspathic
rocks appears to be, that they have been stretched, whilst slowly
flowing onwards in a pasty condition, in precisely the same manner, as
Professor Forbes believes, that the ice of moving glaciers is stretched
and fissured. In both cases, the zones may be compared to those in the
finest agates; in both, they extend in the direction in which the mass
has flowed, and those exposed on the surface are generally vertical. In
the ice, the porous laminæ are rendered

1 James David Forbes (1828-1876), professor of natural philosophy, Edinburgh University, 1833-1860, secretary of the Royal Society of Edinburgh, 1840-1851. This item was reprinted verbatim in Forbes 1859. See Correspondence vol. 3, pp. 66; 74.

throw out the idea, that the structure in question may perhaps be
explained by your views on the zoned structure of glacier ice, the
layers of less tension being, in the case of the Ascension
obsidian-rocks, rendered apparent, chiefly by the crystalline and
concretionary action superinduced in them, instead of, as in zoned ice,
by the congelation of water. * * *

"How singular it at first appears, that your discoveries in the
structure of glacier ice should explain the structure, as I fully
believe they will, of many volcanic masses. I, for one, have for years
been quite confounded whenever I thought of the lamination of rocks
which have flowed in a liquified state. Will your views throw any light
on the primary laminated rocks? The laminæ certainly seem very
generally parallel to the lines of disturbance and movement. Believe
me, &c.
C. DARWIN."

To Professor FORBES.

Professor Forbes confirmed the previous remarks by others, made by
himself on the specimens transmitted to him by Mr Darwin, and on
specimens from Lipari and Iceland in the collection of the Royal
Society, as well as by direct observations made by himself on the lava
streams of Ætna.

distinct by the subsequent congelation of infiltrated
water; in the stony feldspathic lavas by subsequent crystalline and
concretionary action. The fragment of glassy obsidian in Mr Stokes's
collection, which is zoned with minute air-cells, must strikingly
resemble, judging from Professor Forbes's description, a fragment of
the zoned ice; and if the rates of cooling and the nature of the mass
had been favourable to its crystallization, or to concretionary action,
we should here have had the finest parallel zones of different
composition and texture. In glaciers, the lines of porous ice and of
minute crevices seem to be due to an incipient stretching, caused by
the central parts of the frozen stream moving faster than the sides and
bottom, which are retarded by friction. Hence, in glaciers of certain
form, and towards the lower end of most glaciers, the zones become
horizontal. May we venture to suppose that, in the feldspathic lavas
with horizontal laminæ, we see an analogous case? All geologists who
have examined trachytic regions have come to the conclusion, that the
lavas of this series have possessed an exceedingly imperfect fluidity;
and as it is evident that only matter thus characterized would be
subject to become fissured, and to be formed into zones of different
tensions, in the manner here supposed, we probably see the reason why
augitic lavas, which appear, generally, to have possessed a higher
degree of fluidity, are not, like the feldspathic lavas, divided into
laminæ of different composition and texture. Moreover, in the augitic
series, there never appears to be any tendency to that kind of
concretionary action, which, we have seen, plays an important part in
the lamination of rocks of the trachytic series, or, at least, in
rendering that structure apparent."